Image recording systems write digital data onto photosensitive media by apply light exposure energy. Such energy may originate from a number of different sources and may be modulated in a number of different ways. Image recording systems can be used for digital printing, whereby digital image data is used to print an image onto photographic paper or film.
One of the early methods used for digital printing was cathode ray tube (CRT) based systems. In a CRT-based printer, the digital data is used to modulate the CRT, which provides exposure energy by scanning an electron beam of variable intensity along its phosphorescent screen. This technology has several limitations related to the phosphor and the electron beam. The resolution of this technology is inadequate when printing a large format image, such as 8 inch by 10 inch photographic print. CRT printers also tend to be expensive, which is a severe shortcoming in cost sensitive markets such as photoprocessing and film recording. An additional limitation is that CRT printers do not provide sufficient red exposure to the media when operating at frame rates above 10,000 prints per hour.
Another commonly used approach to digital printing is the laser-based engine shown in U.S. Pat. No. 4,728,965. Digital data is used to modulate the duration of laser on-time or intensity as the beam is scanned by a rotating polygon onto the imaging plane. Such raster scan systems use red, green, and blue lasers. Unfortunately, as with CRT printers, the laser based systems tend to be expensive, since the cost of blue and green lasers remains quite high. Additionally, compact lasers with sufficiently low noise levels and stable output so as to allow for accurate reproduction of an image without introducing unwanted artifacts are not widely available.
One problem with laser based printing systems is that both photographic paper and film are not suitable for a color laser printer due to reciprocity failure. High intensity reciprocity failure is a phenomenon by which both photographic paper and film are less sensitive when exposed to high light intensity for a short period. For example, raster scan laser printers expose each of the pixels for a fraction of a microsecond, whereas optical printing systems expose the paper for the duration of the whole frame time, which can be on the order of seconds. Thus, special paper and film are required for laser printers.
In an effort to reduce cost and complexity of printing systems, as well as avoid reciprocity failure, alternative technologies have been considered for use in digital printing. Among suitable candidate technologies under development are two-dimensional spatial light modulators.
Two-dimensional spatial light modulators, such as the digital micromirror device (DMD) from Texas Instruments, Dallas, Tex., or liquid crystal devices (LCD) can be used to modulate an incoming optical beam for imaging. A spatial light modulator can be considered essentially as a two-dimensional array of light-valve elements, each element corresponding to an image pixel. Each array element is separately addressable and digitally controlled to modulate light by transmitting or by blocking transmission of incident light from a light source by affecting the polarization state of light. Polarization considerations are, therefore, important in the overall design of support optics for a spatial light modulator.
There are two basic types of spatial light modulators in current use. The first type developed was the transmission spatial light modulator, which, as its name implies, operates by selective transmission of an optical beam through individual array elements. The second type, a later development, is a reflective spatial light modulator. As its name implies, the reflective spatial light modulator, operates by selective reflection of an optical beam through individual array elements. A suitable example of a LCD reflective spatial light modulator relevant to this application utilizes an integrated CMOS backplane, allowing a small footprint and improved uniformity characteristics.
Spatial light modulators provide significant advantages in cost, as well as avoiding reciprocity failure. Spatial light modulators have been proposed for a variety of different printing systems, from line printing systems such as the printer depicted in U.S. Pat. No. 5,521,748, to area printing systems such as the system described in U.S. Pat. No. 5,652,661.
A single spatial light modulator such as a Texas Instruments digital micromirror device (DMD) as shown in U.S. Pat. No. 5,061,049 can be used for digital printing applications. One approach to printing using the Texas Instruments DMD, shown in U.S. Pat. No. 5,461,411, offers advantages such as longer exposure times than using light emitting diodes (LED) as a source. Thus, the reciprocity problems associated with photosensitive media during short periods of light exposure are eliminated. However, DMD technology is both expensive and not widely available. Furthermore, DMDs are not easily scaleable to higher resolutions, and the currently available resolution is not sufficient for all digital printing needs.
Several photographic printers using commonly available LCD technology are described in U.S. Pat. Nos. 5,652,661; 5,701,185; and 5,745,156. Most of these designs involve the use of a transmissive LCD modulator such as those depicted in U.S. Pat. Nos. 5,652,661 and 5,701,185. While such methods offer several advantages in ease of optical design for printing, there are several drawbacks to the use of conventional transmissive LCD technology. Transmissive LCD modulators generally have reduced aperture ratios and the use of Transmissive Field-Effect-Transistors (TFT) on glass technology does not promote the pixel to pixel uniformity desired in many printing and film recording applications. Furthermore, in order to provide large numbers of pixels, many high resolution transmissive LCDs possess footprints of several inches. Such a large footprint can be unwieldy when combined with a lens designed for printing or film recording applications. As a result, most LCD printers using transmissive technology are constrained to either low resolution or small print sizes.
To print high resolution 8 inch by 10-inch images with at least 300 pixels per inch requires 2400 by 3000 pixels. Similarly, to print high resolution images onto film requires at least 2000 by 1500 pixels, and can require as much as 4000 by 3000 pixels. Transmissive LCD modulators with such resolutions are not readily available. Furthermore, each pixel must have a gray scale depth to render a continuous tone print uniformly over the frame size, which is not available with this technology.
The use of a reflective LCD serves to significantly reduce the cost of the printing system. Furthermore, the use of an area reflective LCD modulator sets the exposure times at sufficient length to avoid or significantly reduce reciprocity failure.
The progress in the reflective LCD device field made in response to needs of the projection display industry have provided opportunities in printing applications. Thus, a reflective LCD modulator designed for projection display can be incorporated into a printing design with little modification to the LCD itself. Also, by designing an exposure system and data path with an existing projection display device allows incorporation of an inexpensive commodity item into the print engine.
Of the reflective LCD technologies, the most suitable to this design is one that incorporates a small footprint with an integrated Complementary Metal Oxide Semiconductor (CMOS) backplane. The compact size along with the uniformity of drive offered by such a device will translate into better image quality than other LCD technologies. There has been progress in the projection display industry towards incorporating a single reflective LCD, primarily because of the lower cost and weight of single device systems. See U.S. Pat. No. 5,743,612. Of the LCD technologies, the reflective LCD with a silicon backplane can best achieve the high speeds required for color sequential operation. While this increased speed may not be as essential to printing as it is for projection display, the higher speeds can be utilized to incorporate additional gray scale and uniformity correction to printing systems.
The recent advent of high resolution reflective LCDs with high contrast, greater than 100:1, presents possibilities for printing that were previously unavailable. See U.S. Pat. Nos. 5,325,137 and 5,805,274. Specifically, a printer may be based on a reflective LCD modulator illuminated by a lamp, lasers, or by an array of red, green, and blue light emitting diodes. The reflective LCD modulator may be sub-apertured and dithered in two or three directions to increase the resolution.
Reflective LCD modulators have been widely accepted in the display market. Most of the activity in reflective LCD modulators has been related to projection display, such as is disclosed in U.S. Pat. No. 5,325,137. Several projector designs use three reflective LCD modulators, one for each of the primary colors. One such design is shown in U.S. Pat. No. 5,743,610.
It is instructive to note that imaging requirements for projector and display use (as is typified in U.S. Pat. Nos. 5,325,137; 5,808,800; and 5,743,610) differ significantly from imaging requirements for digital printing onto photographic paper or film. Projectors are optimized to provide maximum luminous flux to a screen, with secondary emphasis placed on characteristics important in printing, such as contrast and resolution. To achieve the goals of projection display, most optical designs use high intensity lamp light sources. Optical systems for projector and display applications are designed for the response of the human eye, which, when viewing a display, is relatively insensitive to image artifacts and aberrations and to image non-uniformity, since the displayed image is continually refreshed and is viewed from a distance. However, when viewing printed output from a high-resolution printing system, the human eye is not nearly as forgiving to artifacts and aberrations and to non-uniformity, since irregularities in optical response are more readily visible and objectionable on printed output. In fact, the gamma of the human eye, when viewing images in a darkened room is approximately 0.8. The gamma associated with a paper print may be 1.6, thus rendering small artifacts more easily visible in printed images. For this reason, there can be considerable complexity in optical systems for providing uniform exposure energy for printing. Even more significant are differences in resolution requirements. Additionally, projectors are often designed to present motion images. When an image is moving, the presence of defects and artifacts may be dynamic. Between the varying image content and the motion of the artifacts themselves, image variations may not be easily perceived by the human eye. However, when the artifacts are stationary as is the surrounding image data, image quality requirements become more stringent.
A preferred approach for digital printing onto photographic paper and film uses a reflective LCD-based spatial light modulator. Liquid crystal modulators can be a low cost solution for applications requiring spatial light modulators. Photographic printers using commonly available LCD technology are disclosed in U.S. Pat. Nos. 5,652,661; 5,701,185 (Reiss et al.), and U.S. Pat. No. 5,745,156 (Federico et al.). Although the present invention primarily addresses use of reflective LCD spatial light modulators, references to LCDs in the subsequent description can be generalized, for the most part, to other types of spatial light modulators, such as the previously noted Texas Instruments DMD device.
Primarily because of their early development for and association with screen projection of digital images, spatial light modulators have largely been adapted to continuous tone (contone) color imaging applications. Unlike other digital printing and film recording devices, such as the CRT and laser-based devices mentioned above that scan a beam in a two-dimensional pattern, spatial light modulators image one complete frame at a time. Using a LCD, the total exposure duration and overall exposure energy supplied for a frame can be varied as necessary in order to achieve the desired image density and to control media reciprocity characteristics. Advantageously, for printing onto photographic paper and film, the capability for timing and intensity control of each individual pixel allows a LCD printer to provide grayscale imaging.
Modulator printing systems can incorporate a variety of methods to achieve gray scale. Texas Instruments employs a time delayed integration system that works well with line arrays as shown in U.S. Pat. Nos. 5,721,622, and 5,461,410. While this method can provide adequate gray levels at a reasonable speed, line printing Time Delayed Integration (TDI) methods can result in registration problems and soft images. Alternate methods have been proposed particularly around transmissive LCDs such as the design presented in U.S. Pat. No. 5,754,305.
Dithering has been applied to transmissive LCD systems due to the less than perfect fill factor. Incorporating dithering into a reflective LCD printing system would allow high resolution printing while maintaining a small footprint. Also, because the naturally high fill factor present in many reflective LCD technologies, the dithering can be omitted with no detriment to the continuity of the printed image.
Alternative forms of optical dithering are used to improve resolution in display systems incorporating LCD modulators. A calcite crystal or other electro-optic birefringent material can be used to optically shift the path of the image beam, where the amount of shift is dependent on the polarization characteristics of the image. This allows the shifting of one component of the image with respect to a second component of the image that has a different polarization. See U.S. Pat. Nos. 5,715,029 and 5,727,860. In addition to the use of birefringent material, U.S. Pat. No. 5,626,411 uses refraction with isotropic optical media of different indices of refraction to displace one image component from a second image component. These methods of beam displacement are used in a dynamic imaging system and serve to increase resolution by interlacing raster lines to form two lines of sub-images. The two sub-images are imaged faster than is perceivable by the human visual system, so that the individual images are integrated into a composite image as seen by the observer. While these methods are appropriate for projection imaging systems, they are not suitable for a static imaging system such as printing.
While the reflective LCD modulator has enabled low cost digital printing on photosensitive media, the demands of high resolution printing have not been fully addressed. For many applications, such as imaging for medical applications, resolution is critical. Often, the resolution provided by a single reflection LCD modulator is insufficient. It then becomes necessary to create an image wherein multiple images are merged to create a single high-resolution image. Creating a merged image without artifacts along the borders, or in regions where image data may overlap, is desirable. While juxtaposing or spatially interweaving image data alone may have been attempted in previous applications, such a superposition of images with the use of reflective LCDs provides images of high quality without compromising the cost or productivity of the print engine. By utilizing polarization-based modulation, a print engine can utilize light already available in the optical system.
Juxtaposing or spatially interweaving image data has been attempted with some success in projection displays. Fergason, in U.S. Pat. No. 5,715,029, describes a method to improve resolution of a display by altering the beam path using a birefringent medium such as a calcite crystal or an electro-optic liquid crystal cell. For projection applications using a transmissive LCD, Philips Corporation deflects the beam path by using birefringent elements as shown in U.S. Pat. No. 5,727,860. Another method, using isotropic optical elements to juxtapose or spatially interweave images in a projection display using a transmissive LCD, is described in U.S. Pat. No. 5,626,411.
Thus, it is desirable to have a low-cost, high-resolution, high-speed method for digital printing onto a photosensitive media that avoids reciprocity failure and preserves adequate gray scale.